WASHINGTON (AP) — President Barack Obama said he wants more information about chemical weapons use in the Syrian civil war before he decides on escalating U.S. military or diplomatic responses, despite earlier assertions that use of such weapons would be a “game-changer.”

With the U.S. disengaging from the unpopular war in Afghanistan and still smarting from the difficult conflict in Iraq, Obama has been reluctant to unleash American military power in the Syrian fighting, a civil war that has killed tens of thousands of people. The president said at a Tuesday news conference that the conflict is a “blemish on the international community generally.”

Obama said the evidence available does not yet merit the quick use of American military power. He has called for the United Nations to investigate.

“What we now have is evidence that chemical weapons have been used inside of Syria, but we don’t know how they were used, when they were used, who used them. We don’t have a chain of custody that establishes what exactly happened. And when I am making decisions about America’s national security and the potential for taking additional action in response to chemical weapon use, I’ve got to make sure I’ve got the facts. That’s what the American people would expect.

The president said, however, that if it is determined that the regime of Syrian President Bashar Assad used chemical weapons “we would have to rethink the range of options that are available to us.”

In the White House news conference marking the 100th day of his second term, the president said he had a full range of such “options on the shelf.”

Beyond providing weapons and ammunition to the rebels, several government agencies are also drafting plans for establishing a protective “no-fly zone” over Syria and for targeted missile strikes, according to officials familiar with the planning. However, the officials, who spoke only on condition of anonymity because they were not authorized to publicly discuss the internal deliberations, stressed that Obama had not yet decided to proceed on any of the plans.

The problem facing the U.S. is that Syrian air defenses are far stronger than NATO allies faced when they intervened with air power in Libya, and many of the rebel forces are now identified as Islamic radicals, many of them associated with al-Qaida and determined to establish a government based on a strict interpretation of Islamic law.

Noting American humanitarian aid that has flowed into victims of the conflict, Obama said the civil war has been “a slowly unfolding disaster for the Syrian people. And this is not a situation in which we’ve been simply bystanders to what’s been happening. ”

Obama has resisted calls to expand U.S. assistance beyond the nonlethal aid the government is providing the rebels. That has frustrated some allies as well as some U.S. lawmakers, who say the deaths of 70,000 Syrians should warrant a more robust American response.

Polling suggests war-weary Americans are reluctant to see the U.S. get involved in another conflict in the Middle East. A CBS News/New York Times poll out Tuesday shows 62 percent of Americans say the country does not have a responsibility to intervene in the fighting in Syria, while 24 percent say the government does have that responsibility.

Obama also said he would “re-engage with Congress” on the future of the prison for detainees at Guantanamo in Cuba. As a candidate for the White House in 2007 and 2008, Obama called for closing the base, which was set up as part of President George W. Bush’s response to the terror attacks on Sept. 11, 2001. Lawmakers objected and the facility remains open.

Questioned about a hunger strike by some detainees, he said, “I don’t want these individuals to die,” and he said the Pentagon was doing what it could to manage the situation.

Obama also noted that several suspected terrorists have been tried and found guilty in U.S. federal courts, an answer to his congressional critics who maintain that detainees must be tried in special courts if the United States is to maximize its ability to prevent future attacks.